1. Field of the Invention
The present invention relates to a power supply technique for an electronic device.
2. Description of the Related Art
Battery-driven devices such as cellular phone terminals, tablet terminals, laptop computers, and portable audio players each include a rechargeable secondary battery and a charger circuit that charges the secondary battery as built-in components. Known examples of such charger circuits include: an arrangement that charges a secondary battery using a DC voltage (bus voltage VBUS) supplied from an external circuit via a USB cable; and an arrangement in which the secondary battery is charged using the DC voltage supplied from an external AC adapter.
At present, as a charger circuit mounted on a mobile device, charger circuits that conform to a specification which is referred to as the “USB Battery Charging Specification” (which will be referred to as the “BC specification” hereafter) have become mainstream. There are several kinds of USB hosts or USB chargers (which will collectively be referred to as a “USB power supply apparatus” hereafter). As the kinds of USB power supply apparatuses that conform to revision 1.2 of the BC specification, SDP (Standard Downstream Port), DCP (Dedicated Charging Port), and CDP (Charging Downstream Port) have been defined. The current (current capacity) that can be provided by a USB power supply apparatus is determined according to the kind of USB power supply apparatus. Specifically, DCP and CDP are defined to provide a current capacity of 1500 mA. Also, SDP is defined to provide a current capacity of 100 mA, 500 mA, or 900 mA, according to the USB version.
As a next-generation secondary battery charging method or system using USB, a specification which is referred to as the “USB Power Delivery Specification” (which will be referred to as the “PD specification” hereafter) has been developed. The PD specification allows the available power to be dramatically increased up to a maximum of 100 W, as compared with the BC standard, which provides a power capacity of 7.5 W. Specifically, the PD specification allows a USB bus voltage that is higher than 5 V (specifically, 9 V, 12 V, 15 V, 20 V, or the like). Furthermore, the PD specification allows a charging current that is greater than that defined by the BC specification (specifically, the PD specification allows a charging current of 2 A, 3 A, 5 A, or the like). The PD specification is also defined in the USB Type-C specification.
FIG. 1 is a block diagram showing a power supply system 100R investigated by the present inventor. The power supply system 100R conforms to the USB Type-C specification, and includes a power supply apparatus 200R and a power receiving apparatus 300R coupled via a USB cable 106. For example, the power supply apparatus 200R is mounted on an AC adapter 102, or is mounted on an electronic device. The power receiving apparatus 300R is mounted on a battery-driven electronic device 400 such as a smartphone, a tablet terminal, a digital still cameral, a digital video camera, a portable audio player, or the like.
The power supply apparatus 200R includes a power supply circuit 202 and a power supply side PD controller (which will be referred to as the “power supply side controller” hereafter) 204. A SOURCE PDO (Power Data Object) list or table (which will also be referred to simply as the “PDO list” hereafter) 206 is capable of holding a maximum of seven defined combinations (PDOs) of the voltage and current that can be supplied by the power supply apparatus 200R.
The USB cable 106 is detachably coupled to a receptacle 108 of the electronic device 400. It should be noted that such a receptacle 108 may be omitted. That is to say, charger adapters are known having a configuration in which the USB cable 106 and the AC adapter 102 are monolithically integrated.
The receptacle 108 includes a VBUS terminal configured to supply a bus voltage VBUS, a GND terminal configured to supply a ground voltage VGND, and a CC (Configuration Channel) terminal. With receptacle-type terminals, two CC terminals are provided. However, in the present embodiment, one terminal is omitted for simplicity of description. The power supply circuit 202 generates the bus voltage VBUS. The power supply circuit 202 may include an AC/DC converter that receives an AC voltage of 100 V from an unshown external power supply (e.g., a commercially available AC power supply), and that converts the AC voltage thus received into the bus voltage VBUS in the form of a DC voltage. The bus voltage VBUS generated by the power supply circuit 202 is supplied to the power receiving apparatus 300R via a bus line of the USB cable 106.
The power supply side controller 204 is coupled to a power receiving side PD controller (which will be referred to as the “receiving side controller” hereafter) 306 via the USB cable 106. The power supply side controller 204 and the power receiving side controller 310 provide a communication function that allows communication between the power supply apparatus 200R and the power receiving apparatus 300R.
The electronic device 400 incudes a load (system) 402 in addition to the power supply apparatus 300R. Examples of such a load 402 include CPUs, memory, liquid crystal displays, audio circuits, and the like. The AC adapter 102 is detachably coupled to the receptacle 404 via the USB cable 106.
The power receiving apparatus 300R includes a battery 302, a charger circuit 304, a power supply circuit 306, a DC/DC converter 308, a power receiving side controller 310, and a SINK PDO list or table (which will also be referred to simply as the “PDO list” hereafter) 312.
The battery 302 is a rechargeable secondary battery. The charger circuit 304 receives the bus voltage VBUS (which will also be referred to as the “adapter voltage VADP” on the power receiving apparatus 300R side) from the power supply apparatus 200R via the USB cable 106, so as to charge the battery 302. The charger circuit 304 is configured as a step-down DC/DC converter, a linear regulator, or a combination of such components.
A system voltage VSYS is supplied from the charger circuit 304 to the load 402 according to at least one of the adapter voltage VADP and the voltage VBAT supplied from the battery 302. Examples of such a load 402 include power management ICs (Integrated Circuits), multi-channel power supplies each including a DC/DC converter, linear regulator or the like, microcomputers, liquid crystal displays, display drivers, and so forth.
A load 402 operates with a system voltage VSYS of 20 V and a maximum current of 2.25 A (with an electric power of 45 W), for example. Also, the electronic device 400 supports a bus voltage VBUS of 15 V in addition to a bus voltage VBUS of 20 V. In the PDO list 312, combinations of the voltage and the current to be requested by the electronic device 400 are each defined as a PDO. In this example, a combination of 20 V and 2.25 A and a combination of 15 V and 3 A are each defined as a PDO. Furthermore, in the USB-PD specification, the power supply side and the power receiving side are both required to support a voltage of 5 V (without a definition of the current).
In order to satisfy such a request, the power supply circuit 306 is provided as an upstream stage of the charger circuit 304. The power supply circuit 306 includes the DC/DC converter 308. In a case in which VBUS=15 V, the DC/DC converter 308 steps up the bus voltage VBUS so as to generate an adapter voltage VADP of 20 V.
The power receiving controller 310 is configured as a port controller that conforms to the USB Type-C specification. The power receiving controller 310 is coupled to the power supply side controller 204 via a CC line. After the AC adapter 102 and the electronic device 400 are coupled, negotiation is performed between the power supply side controller 204 and the power receiving side controller 310. As a result, the voltage level of the bus voltage VBUS is determined based on the PDO list 206 and the PDO list 312.
FIG. 2 is a sequence diagram showing a negotiation operation that conforms to the USB-PD specification shown in FIG. 1. First, the power supply side controller 204 transmits the PDO list 206 to the power receiving side controller 310, so as to declare the voltage/current combinations that can be supplied (which will be referred to as “Source Capability”) (S100).
The power receiving side controller 310 refers to the PDO list 312, and selects a single optimum PDO from the PDO list 206 (S102). Subsequently, the power receiving side controller 310 transmits an RDO (Request Data Object) including the selected PDO and the amount of current to be consumed (S104). If the amount of current that can be supplied from the power supply apparatus 200R is smaller than the amount of current requested by the power receiving apparatus 300R, the RDO includes information indicating that a mismatch has occurred between the source capability and the receiver request. That is to say, in this case, the power receiving apparatus 300R transmits, to the power supply apparatus 200R, a notice that only a suboptimal PDO has been selected based on the negotiation.
After the power supply side controller 204 receives the RDO, the power supply side controller 204 sets the output voltage VBUS of the power supply circuit 202 to a value indicated by the PDO (S106). It should be noted that the current requested by the PDO list 312 does not necessarily match the current values defined in the PDO list 206.
Description will be made regarding an example in which the power supply apparatus 200R supports four PDOs, i.e., PDO1 through PDO4 defined in the PDO list 206 shown in FIG. 1. Directing attention to the power receiving side apparatus 300R side, voltage conversion by means of the DC/DC converter 308 involves power loss. Accordingly, a bus voltage VBUS of 20 V is preferably selected, which allows the DC/DC converter 308 to operate with high efficiency. That is to say, the power receiving side controller 310 selects the PDO4, which indicates a combination of 20 V and 2.25 A, from the PDO list received in Step S100. Furthermore, the power receiving side controller 310 transmits an RDO including the PDO4 and the current value of 2.25 A to be consumed in the electronic device 400. In this case, the power supply system 100 operates effectively.
As a result of investigating such a power supply system 100R shown in FIG. 1, the present inventor has come to recognize the following problem.
The number of PDOs that can defined in the PDO list 206 is limited. Accordingly, it is not necessarily the case that all the voltage/current combinations that can be generated by the power supply circuit 202 are defined in the PDO list 206. In the example shown in FIG. 1, a situation is possible in which the PDO list 206 does not include the PDO4, although the power supply circuit 202 supports the voltage/current combination of 20V and 2.25 A.
In this case, the power receiving side controller 310 selects the PDO3 from among PDO1 through PDO3. As a result, there is a need to operate the DC/DC converter 308, leading to degraded efficiency.
Also, when the power receiving apparatus 300R requests the voltage/current combination of 19 V and 2.38 A (i.e., 45 W) instead of the voltage of 20 V, the power receiving side controller 310 selects the PDO3 even if the PDO list 206 includes the PDO4 (20 V/2.25 A). Such a case also leads to degraded efficiency.
In some cases, the power supply system 100R can become inoperable depending on the combination of the PDO list 206 and the PDO list 312, which is also a conceivable problem.
It should be noted that such a problem is not restricted to such a USB-PD specification, but can occur in various kinds of power supply systems employing similar protocols.